Multi-mode interference coupler-based flat compressive and transform imager

ABSTRACT

A compressive/transform imager comprising a lens array positioned above input ports for collecting light into the input ports, waveguides routing the light from the input port to waveguide mixing regions (e.g. multi-mode interference couplers), and detectors for receiving outputs of the waveguide mixing regions.

RELATED APPLICATIONS

This application claims priority from and the benefit of U.S.Provisional Patent Application No. 62/578,120 filed on Oct. 27, 2017,the entire contents of which are incorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support awarded by the DefenseAdvanced Research Projects Agency. The government has certain rights inthe invention.

TECHNICAL FIELD

The present disclosure relates generally to compressive imagers; andmore particularly to multi-mode interference coupler-based flat,compressive and transform imagers.

BACKGROUND

Imaging systems for many applications are required to have minimal sizeand weight, for example for star trackers, smartphone cameras, etc.However, this presents challenges since imaging systems are generallybulky because they require a sensor and lens placed approximately at afocal length away to form an image. Increasing the light collection ofthe imaging system requires increasing the diameter of the lens, whichrequires a proportional increase in the focal length. This makes itdifficult to reduce the size of imaging systems.

For applications that require a minimal size imaging system, it isdesirable to have a flat/chip-scale imager having a thickness ofapproximately 1 mm. A possible configuration for a flat imager can bemade by fabricating a Butler matrix or H-tree using multiple waveguides.An H-tree arrangement can maintain nearly equal path lengths for allpaths between couplers and detectors. Such approaches have been usedextensively for radio frequency (RF) beam forming and imaging. However,these configurations generally have complicated waveguide routing, whichoften requires multiple layers of fabrication and vertical interconnectsor couplers. Multilayer fabrication and vertical interconnects are moredifficult to implement in optical systems than RF systems due to featuresize and material limitations. Also, these configurations are verysensitive to phase differences that are introduced by fabricationtolerances due to the comparatively shorter optical wavelengths. Sometypes of flat imagers can require a large number of waveguides. This canrequire multiple chips arranged in three dimensions for waveguiderouting. A flat imager having a small number of waveguides, a minimalnumber of or no vertical interconnects, and minimal sensitivity tovariations introduced by fabrication tolerances is easier to build andcan potentially occupy a smaller volume.

SUMMARY

At least one embodiment of the present invention is directed to a flatimager design, which can be based on a single layer of waveguidefabrication, which greatly simplifies the design and fabricationprocess. The imager comprises a compressive/transform sensor and aphased array of optical couplers.

At least one embodiment of the present invention is directed to a flatimager design, which can receive inputs from many directions at the sametime. By recovering many scrambled outputs, the imager can localize thedirection of arrival of incoming light without the need for a lens orfocusing optic placed a focal length in front of the detector.Randomness between the outputs ensures that all outputs contributeunique information to the image and is required to ensure no blind spotsin the field of view. Random mixing of inputs makes the embodiments ofthe present disclosure less sensitive to path length mismatch betweendifferent input ports in the device, compared to the path matchingrequirements of an H-tree. This greatly simplifies fabrication.

In another example embodiment, the present disclosure relates to atransform imager including a chip, a number of input ports formed in thechip for coupling light to a waveguide array; a waveguide mixing regionconfigured to receive light from the waveguide array; and a number ofdetectors for receiving outputs from the waveguide mixing region. Insome embodiments, the transform imager is a compressive transform imagerwherein some outputs from the waveguide mixing region are terminated,such that the number of input ports is greater than the number ofdetectors. In another embodiment, the imager also includes a micro-lensarray formed on the chip for collecting light into the first pluralityof input ports. In another embodiment, the imager also includes arandomly spaced, sparsely spaced, or non-uniformly spaced input arrayfor collecting light into the first plurality of input ports. In anotherembodiment, the input ports include a grid of N×M regularly spaced inputports. In another embodiment, the waveguide array is a multi-mode H-treearray configured to maintain substantially equal path lengths betweenthe first plurality of input ports and the second plurality ofdetectors. In another embodiment, the waveguide array and waveguidemixing region are made of a material selected from the group consistingof a silicon, silicon nitride, sapphire, titanium dioxide, diamond,silicon carbide, chalcogenide glass, and plasmonic structures. Inanother embodiment, each of the first plurality of input ports is anaperture or a grating coupler. In another embodiment, the waveguidearray and a number of initial waveguide mixing regions are formed in aplurality of layers, wherein some outputs of the initial waveguidemixing regions are terminated. In another embodiment, the terminatedoutputs of the initial waveguide mixing region are randomly orirregularly spaced. In another embodiment, the imager also includes afinal waveguide mixing region configured to receive non-terminatedoutputs from the third plurality of waveguide mixing regions andscramble information from the non-terminated outputs before couplingthem to the second plurality of detectors. In another embodiment, thefinal waveguide mixing region is a compressive imager wherein at leastsome of the outputs of the final waveguide mixing region are terminatedprior to detection by second plurality of detectors. In anotherembodiment, the waveguide mixing region is a multi-mode Rotman lensconfigured to focus light received from the first plurality of inputports.

In another non-limiting example, the present disclosure relates to amethod of extracting an image from a transform imager. The methodincludes receiving light from a specific image input at a firstplurality of input ports of a transform imager; directing the light to awaveguide mixing region via a waveguide array; mixing the light withinthe waveguide mixing region; receiving mixed light at a second pluralityof detectors from the waveguide mixing region; measuring responses fromthe second plurality of detectors to a specific image input in order toperform calibration; selecting measured responses corresponding to peaksin the image input; and adjusting the calibration in order to recoverdifferent targeted information. In some embodiments, selecting measuredresponses includes identifying significant components of the measuredresponses including the top 5% of values. In some embodiments, thetransform imager is a compressive transform imager wherein some outputsfrom the waveguide mixing region are terminated, such that the firstplurality of input ports has a number greater than the second pluralityof detectors. In some embodiments, the method also includesreconfiguring an output response of a compressive/transform imager by:introducing a phase shift between −π to π or between 0 to 2π at inputapertures; and recovering additional information to recover a morecomplex scene using a unique set of output responses from the imager.

In another non-limiting example, the present disclosure relates to amethod for calibrating a transform imager. The method includesdetermining a complex matrix representing a modification of phase oramplitude to a transfer matrix of the transform imager; determining aset of unique outputs from the transform imager; recovering an initialtransfer matrix of the transform imager with no modifications of phaseor amplitude introduced; combining the initial transfer matrix and thecomplex matrix representing the modification of phase or amplitude; andrecovering additional information from the imager by using N times moreunique modifications of phase or amplitude at inputs to account for an Ntimes reduction in total number of output ports.

In another non-limiting example, the present disclosure relates to amethod of making a compressive transform imager having n number of inputports suitable for detecting a number of expected features k in a scene.The method includes forming a chip having n input ports formed on thechip for coupling light to a waveguide array; forming a waveguide mixingregion configured to receive light from the waveguide array; coupling aplurality of detectors to the waveguide mixing region for receivingoutputs from the waveguide mixing region; and terminating some of theoutputs from the waveguide mixing region, wherein a number of terminatedoutputs is proportional to k*log(n/k).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an overview of the components of at least one embodiment ofa compressive/transform imager of the present disclosure.

FIG. 2 shows another embodiment of a compressive imager of the presentdisclosure.

FIG. 3 shows an example of a multi-mode interference coupler, accordingto an embodiment of the present disclosure.

FIG. 4 shows an example of a broadband multi-mode interference coupler,according to an embodiment of the present disclosure.

FIGS. 5A-5B show the transmission spectrum of a standard MMI and abroadband MMI, according to an embodiment of the present disclosure.

FIG. 6 shows an overview of the components of a flattened 2Dcompressive/transform imager, according to an embodiment of the presentdisclosure.

FIG. 7 shows the specifications for the flattened 2Dcompressive/transform imager of FIG. 6, according to an embodiment ofthe present disclosure.

FIGS. 8A-8B show the output waveguide routing of a multi-layer device,according to an embodiment of the present disclosure.

FIGS. 9A-9C show the processing to convert detector response of a flat2D sensor to a 1D image value (e.g. star location), according to anembodiment of the present disclosure.

FIG. 10 shows an overview of the components of a wide field-of-view(FOV) compressive/transform imager, according to an embodiment of thepresent disclosure.

FIG. 11 shows the specifications for the wide field-of-view (FOV)compressive/transform imager of FIG. 10, according to an embodiment ofthe present disclosure.

FIGS. 12A-12B show a schematic view of a sample system forcharacterizing the compressive/transform imager, according to anembodiment of the present disclosure.

FIG. 13 is a representative schematic of an array of 8×8 input ports,each of which has a phase shifter on each aperture capable of a phaseshift of 0-2π, according to an embodiment of the present disclosure.

FIG. 14 shows simulation results of a random imager which makes use ofre-phasing, according to an embodiment of the present disclosure.

FIG. 15A-15C show a schematic view of sample Rotman lenses used as thewaveguide mixing region in a compressive/transform imager, according toan embodiment of the present disclosure.

FIGS. 16A-16C show schematics of a single Rotman lens used for a 2Daperture array, according to embodiments of the present disclosure.

FIG. 17 shows a schematic of a multi-Rotman lens system, according to anembodiment of the present disclosure.

FIG. 18 shows a schematic of another multi-Rotman lens system, accordingto an embodiment of the present disclosure.

FIG. 19 shows a schematic of another multi-Rotman lens system used fordetermining light source location, according to an embodiment of thepresent disclosure.

FIGS. 20A-20B show two schematics of input and output spacing forexample Rotman lenses, according to an embodiment of the presentdisclosure.

FIGS. 21A-21B show intensity patters for two Rotman lens devices,according to embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the invention can include an integrated, on-chipcompressive/transform imager comprising input ports/apertures connectedto waveguides. The waveguides route light to waveguide mixing regions(e.g. multi-mode interference couplers, MMI's). These couplers routelight to output ports connected to or routed to detectors. Embodimentsof the invention also can include methods of extracting an image fromthe scrambled signal at the output ports of the device, and methods ofcorrecting for the spectrum of the signal and determining the spectrumof the signal using the compressive/transform imager described herein.Embodiments of the invention also include methods for increasing theamount of information that can be extracted from an imager with alimited number input ports/apertures. These techniques enable theinvention to be developed for general imaging applications (i.e. imagingand recovering complex scenes).

Throughout the specification, the term “wafer-scale imager” means anyimaging system built by patterning a waveguide array onto a wafer orintegrated photonic chip. The term “phased array” means an arrangementof apertures/collectors connected so as to act as a single, larger,aperture or collector. The term “transform imager” means an imagingsystem that transforms the input into an output using a well-definedmathematical transform, e.g. Fourier, FFT, DCT, Hadamard transform, orrandom. The term “compressive imager” means an imaging system thatdiscards some of the incoming light, or doesn't fully sample all of theincoming information. One skilled in the art will realize that acompressive imager performs a mathematical transform, while also being“compressive” in that some of the incoming light or information isdiscarded. However, because the light has been mixed before dropping offone or more modes or terminating some of the outputs, some informationfrom each of the inputs is still preserved.

FIG. 1 shows an overview of the components of at least one embodiment ofa compressive/transform imager of the present disclosure. As shown inFIG. 1, this embodiment includes a micro-lens array 101 b, which ispositioned above input ports 110 c, which can consist of apertures orgrating couplers. The micro-lens array 101 b can be used to increase thelight collection of the device. The device can be fabricated directly ona chip 101 a. Light is coupled into a waveguide array 101 d, and theninto a waveguide mixing region 101 e (e.g., multi-mode interferencecouplers, MMI's). Outputs from the MMI's can be fed directly intodetectors 101 f positioned at the output of the device. Different typesof MMI's can be used. For example, narrow-band MMI's can be used whenthe input signal is a narrow-band signal, and broad-band MMI's can beused to improve the throughput of the imager when the input signal is abroadband signal. In some example embodiments, the compressive/transformimager may not require a micro-lens array 101 b, and may instead includea random array of couplers formed on the chip 101 a. For example, theimager may be implemented with a randomly spaced, sparsely spaced, ornon-uniformly spaced input array for collecting light into the imager.

FIG. 2 shows how several layers of MMI's 102 a can be used to combinethe input from a large number of input ports. A fraction of the outputports of the MMI can be terminated. This reduces the number of totalwaveguides routed to output ports, which improves the ease of routingwaveguides on the chip and reduces the total number of detectorsrequired at the output. Terminating fewer output ports from MMIsincreases the total signal collected by the device. Additionally, forcomplex images, each output port contributes approximately one bit ofinformation. Therefore, a larger number of output ports is required formore complex scenes. In FIG. 2, four (4) out of eight (8) output portsare terminated, but any fraction of output ports can be terminated. Theports that are dropped can be randomly selected in order to minimize thecorrelation of output intensity between the final output ports of thedevice. Adding a random phase shift to the individual input ports isanother option for decreasing the correlation of the signal betweendifferent output ports. A final MMI 102 b can be used to scramble thesignal from all waveguides on the device before coupling the MMI's tothe detectors at the output. This makes it possible to localize thedirection of arrival of incoming star light in a faster and moreefficient manner. Some output ports at the final MMI 102 b can bedropped. Alternatively, all outputs of the final MMI 102 b can be routedto the detectors. The final MMI 102 b can be designed with a largernumber of input and output ports in order to increase the number ofoutput ports for the device, which increases the total number ofindependent measurements that can be made by the device. The totalnumber of output ports impacts the accuracy to which acompressive/transform imager can recover the location of a star whenutilized as a star tracker and, more generally, the complexity of thescene that can be recovered by the compressive/transform imager. Agreater number of output ports enables more accurate star tracking andrecovery of more complex scenes.

In a multi-mode H-tree arrangement, at each stage of combination, morethan one of the outputs are preserved and carried to the next level.This has the effect of preserving light and information that otherwisewould be discarded. The “compressive” part of the imager, at each stageof combination, discards some of the waveguides. This is necessarybecause otherwise there would be as many outputs required as there aregrating couplers, which would be hard to accommodate in the wafer. Animportant factor is how one chooses to discard some of the waveguides.First, the light is scrambled in the combination stage using amulti-mode interference coupler (MMI), and optionally additionalphases/path delay added to individual waveguides. These are random, soas to maximally spread the light from any given input coupler into manydifferent output couplers. That way, even though some of the outputs arediscarded, some information is preserved from every input. Full recoveryunder certain circumstances is possible (known in the literature as“compressive sensing”) (Canes, E. J., Romberg, J. K., & Tao, T. (2006).Stable signal recovery from incomplete and inaccurate measurements.Communications on Pure and Applied Mathematics, 59(8), 1207-1223.https://doi.org/10.1002/cpa.20124, Candes, E. J., Romberg, J., & Tao, T.(2006). Robust uncertainty principles: exact signal reconstruction fromhighly incomplete frequency information. IEEE Transactions onInformation Theory, 52(2), 489-509.https://doi.org/10.1109/TIT.2005.862083). The example shown in FIG. 2can be called a “staged” multi-mode compressive/transform imager becauseit includes a number of MMIs at various stages in the device.

FIGS. 3 and 4 show some examples of MMI designs that can be used withthe compressive/transform imager. FIG. 3 shows a non-broadband MMI. Asshown in FIG. 3, the MMI includes a plurality of tapered waveguides atthe input and output of the MMI coupler, which can be designed tomaximize the transmission of the compressive/transform imager. The MMIof FIG. 3 includes an MMI core material 103 a, and an MMI claddingmaterial 103 b. The MMI core material 103 a can be made of silicon.Other materials can be used, such as silicon nitride, sapphire, titaniumdioxide, diamond, silicon carbide, chalcogenide glass, which can be usedto extend the wavelengths of into the visible spectrum. In additionalembodiments, the collectors, waveguides, mixers, or other componentsdescribed herein can be implemented with plasmonic structures (i.e.structures implemented by combining metal and dielectric materials). Ina non-limiting example, all of these materials have a sufficiently highrefractive index to work with silica cladding, which makes packagingdevices easier. The MMI cladding material 103 b can be made of silica.Other materials can be used, such as cytop (a fluoropolymer), SU-8,other low index polymers, or any material from the list of materialsabove, so long as the MMI core material chosen has a higher index ofrefraction than the cladding material. The substrate used for devicescan be a silicon chip with thick thermal oxide to ensure light in thewaveguide layer does not interact with the substrate. The MMI design isdefined by an MMI length 103 c, an MMI width 103 d, a waveguide width103 e, a waveguide spacing 103 f, and a tapered waveguide width 103 g.These dimensions determine the transmission characteristics (i.e.,wavelength, efficiency). The thickness of the MMI core material 103 a isalso an important design parameter.

FIG. 4 shows a broadband MMI designed for broadband transmission thatcan be used with embodiments of the present disclosure. Details on onemethod of designing broadband MMIs is given in (Maese-Novo, a, Halir,R., Romero-García, S., Pérez-Galacho, D., Zavargo-Peche, L.,Ortega-Moñux, A., . . . Cheben, P. (2013). Wavelength independentmultimode interference coupler. Optics Express, 21(6), 7033.https://doi.org/10.1364/OE.21.007033). As shown in FIG. 4, the broadbandMMI comprises a plurality of tapered waveguides at the input and outputof the MMI coupler, which can be designed to maximize the transmissionof the compressive/transform imager. The MMI of FIG. 4 also comprises anMMI core material 104 a, and an MMI cladding material 104 b. The MMIcore material 104 a can be made of silicon and the MMI cladding material104 b can be made of silica. The other materials listed above withrespect to FIG. 3 also can be used as the core material and claddingmaterial to extend the wavelengths of into the visible spectrum. The MMIdesign is defined by an MMI length 104 c, an MMI width 104 d, awaveguide width 104 e, a waveguide spacing 104 f, and a taperedwaveguide width 104 g. The MMI of FIG. 4 also comprises sub-wavelengthgrating 104 h alternating between the core and cladding material 104a/104 b. The sub-wavelength grating 104 h forms the tapers and body ofthe broadband MMI. The dimensions and periodicity of the sub-wavelengthgrating 104 h determine the transmission characteristics (i.e.,wavelength, efficiency). The thickness of the MMI core material 104 a isalso an important design parameter.

Appropriate designs for the MMI's of the compressive/transform imagercan be generated by analytical formulas known in the art (e.g., Fullreferences which provide design rules: Bachmann, M., Besse, P. a, &Melchior, H. (1994). General self-imaging properties in N×N multimodeinterference couplers including phase relations. Applied Optics, 33(18),3905-3911. https://doi.org/10.1364/AO.33.003905 Soldano, L. B., &Pennings, E. C. M. (1995). Optical multi-mode interference devices basedon self-imaging: principles and applications. Journal of LightwaveTechnology, 13(4), 615-627. https://doi.org/10.1109/50.372474. Heaton,J. M., & Jenkins, R. M. (1999). General Matrix Theory of Self-Imaging inMultimode Interference (MMI) Couplers. IEEE Photonics TechnologyLetters, 11(2), 212-214. https://doi.org/10.1109/68.740707). Precisedesigns require the use of numerical methods, such as eigenmodeexpansion solvers, finite-difference time-domain methods, and beampropagation methods in order to calculate and optimize the performance.Numerical methods allow rapid testing of many iterations of MMI designsand dimensions to optimize transmission properties in the desiredwavelength ranges. The length of the MMI should be chosen to ensuremaximum mixing of signals from all input ports with one another at thedevice output. This can be achieved by choosing the length of the MMIsuch that it can function as a 1×N splitter (N is the number of inputand output ports), where the signal from each of the N inputs is splitto N outputs.

It is desirable to use the broadband MMI shown in FIG. 4 in order toenable a high throughput of a broad range of wavelengths through thecompressive/transform imager. For example, a broadband MMI with 4 inputports and 4 output ports can have a transmission bandwidth of 350 nm,over 5 times as wide as a non-broadband MMI with 4 input and 4 outputports, as shown in FIGS. 5A-5B. This can be beneficial for acompressive/transform imager that operates with a broadband inputspectrum. Also, improvements to MMI's and grating couplers at the inputports can increase the transmission of light and decrease the size ofthe device.

A variety of designs can be utilized for the integrated opticalcomponents of the compressive/transform imager. For example, thetop-down profile and index profile of the MMI's can be modified tooptimize transmission for different wavelengths, narrow-bandtransmission, and broad-band transmission. Examples of different designsfor broad-band transmissions are presented in the following references:“Broadband 2×2 Multi-mode Interference Coupler for T- and O-bandCommunication by Wavefront Matching Method, 14-16,” Sugiyama, K. &Tsuda, H. (2015), https://doi.org/10.1364/IPRSN.2015.JMA3A.32; “Indexprofile engineering of multimode interference couplers,” New.design,2(x), 76300X-76300X-8. https://dpo.org/10.1117/12.853236, Ortega-Monux,A., Molina-Fernandez, I. & Wanguemert-Perez, J. G. (2009).

FIG. 6 shows an overview of the components of another embodiment of acompressive/transform imager of the present disclosure. As shown in FIG.6, the compressive/transform imager can be a flattened 2D imager, whichcomprises a lens array 600 formed on a light collection array 610,consisting of apertures (e.g. grating couplers) and waveguides 620formed below the light collection array 610. Additional layers ofwaveguides and apertures 620 can be formed below the first lightcollection array 610. A propagation/mixing region 630 is formed adjacentto the lens array 600. A detector array 640 is formed adjacent to thepropagation/mixing region 630. In the compressive/transform imager ofFIG. 6, the light is directed in-plane to a linear array.

FIG. 7 shows details of the compressive/transform imager shown in FIG.6. As shown in FIG. 7, the compressive/transform imager includes a400×400 lens array 700 at a 60 μm spacing and a one degree field ofview. Approximately twenty (20) layers of waveguides 720 are formedbelow the light collection array 710. A propagation/mixing region 730 isformed adjacent to the lens array 700. A detector array 740 is formedadjacent to the propagation/mixing region 730. In thecompressive/transform imager of FIG. 7, the light mixes in thepropagation region 730. Each of the layers in the collection array 710can include all optical elements, e.g. waveguides and grating couplers,to make it independent, removing the need for vertical opticalinterconnects between layers.

FIG. 8A shows a top view of a routing configuration of the array of thelayers of waveguides 720 of FIG. 7. The array of waveguides 720 includea top layer 810, a middle layer 820 and a bottom layer 830. As shown inFIG. 8A, the path lengths are matched inside the array and outputwaveguides from separate layers are stacked vertically with respect toone another. The array can include additional layers in order toaccommodate output waveguide spacing. FIG. 8B shows a side view of onegroup of outputs of the array of waveguides 720. Outputs from one columnof inputs is shown in a cross-sectional view.

FIGS. 9A-9C show how the detector responses are processed to determine astar location. As shown in FIG. 9A, initially, the detector response areexponentially distributed. With an exponential distribution, theresponse as a function of detector number is deterministically random.As shown in FIG. 9B, thresholding yields a unique signature for thedetector responses. In a non-limiting example, the thresholding stepidentifies the significant components of the matrix, which can includethe top 5% of values in the matrix. The matrix shown in FIG. 9B afterthresholding has been performed can be a binary matrix of 1 or 0 values.As shown in FIG. 9C, a look-up table can then be used to determine thelocation of a star based on the unique signature found in FIG. 9B. Thelookup table contains data for the threshold detector responses expectedfor a grid of star locations within the field of view. The star locationis determined by finding the star location that leads to the lowesterror between the lookup table signature and measured signature.

FIG. 10 shows an overview of the components of another embodiment of acompressive/transform imager of the present disclosure. As shown in FIG.10, the compressive/transform imager includes a lens array 1000 formedon a light collection array 1010 to which light is directed. The lensarray 1000 can be moved to scan a field of view. A MEMS actuator 1020for controlling phase shift is formed adjacent the lens array 1010. Apropagation/mixing region 1030 is formed adjacent to the MEMS actuator1020. In a non-limiting example, the MEMS actuator is formed on the samesubstrate as the mixing region. The mixing region allows the light tointerfere in a somewhat random way, similar to the multi-modeinterferometers described earlier in this document. In the case that thelight collection array consists of multiple layers, the mixing regioncan have a larger extent perpendicular to the plane of the device (e.g.micrometers instead of 100s of nanometers, as is typical in manyintegrated photonic devices). A detector array 1040 is formed adjacentto the propagation/mixing region 1030. In the compressive/transformimager of FIG. 10, the light mixes in the propagation region 1030. Thecollection array 1010 can include all optical elements, e.g. waveguidesand grating couplers, to make it independent, removing the need forvertical optical interconnects between layers. In thecompressive/transform imager of FIG. 10, the light is directed in-planeto a linear array.

FIG. 11 shows details of the compressive/transform imager shown in FIG.10 that can be used to process data to locate starts. As shown in FIG.11, the compressive/transform imager includes a 90×90 lens array 1100 ata 250 μm spacing and a 0.25 degree instantaneous field of view. FIG. 11shows one layer of waveguides 1110 directed into the edge of the mixingregion 1120. A propagation/mixing region 1120 is formed adjacent to thelens array 1100. A detector array 1130 is formed adjacent to thepropagation/mixing region 1120. In the compressive/transform imager ofFIG. 11, beam steering is accomplished by a lens array movable in thex/y plane to focus on side lobes of the array of collection apertures,and a MEMS actuator for phase shifting, which accounts for pathdifferences at different viewing angles. A multimode interferometer isused to mix light from inputs.

Before usable data can be extracted from the compressive/transformimager, the imager must be characterized. The characterization processfor using the compressive/transform imager as a star tracker ispresented below. However, as mentioned above, the compressive/transformimager can be used for other applications such as smartphone cameras,etc. i.e. a general compressive/transform imager. The characterizationprocess for a general compressive/transform imager is similar to that ofa compressive/transform imager designed as a star tracker.Characterization should be performed in a basis that the detected imagesare sparse in. For example, a star tracker can effectively be calibratedusing a point source, while a general imager can be calibrated in adiscrete cosine transform, Fourier, Haar, or related basis.

In order to predict the performance of the compressive/transform imager,the wavelength-dependent scattering matrices of all of the MMI's in theimager need to be calculated. This can be done using simulation methodssuch as eigenmode expansion solvers, finite-difference time-domainmethods, and beam propagation methods. The scattering matrices of alldevice components are combined into a device scattering matrix based onthe device design. This device scattering matrix is used to determinethe device output as a function of point source location within thefield of view.

Characterization of the compressive/transform imager can be done byrecording the output of the imager as a point source of light is movedacross a grid of points within the field of view. An example system forcharacterizing the compressive/transform imager is shown schematicallyin FIGS. 12A-12B. Characterization information for thecompressive/transform imager output as a function of point sourcelocation must be recorded for a large number of grid points, “n×n”. FIG.12B shows an example of two representative point source locations 106 dwithin the field of view. The number of points “n×n” that must berecorded for characterization is defined by the number of input portsN×N of the imager. The theoretical minimum number of points that must berecorded corresponds to n=N, but for good performance, it is typicallyrequired for n to be greater than 4×N. This results in a matrix P of“N_(out)×n×n” data points, where N_(out) is the number of output portsof the imager. As shown in FIG. 12A, there are 16×16 input ports 106 aand 32 output ports 106 b. In this example we take “n=12.5×N”, resultingin a calibration matrix P consisting of “32×200×200” data points.

When using the compressive/transform imager as a star tracker lookingfor a single star, the measured output is compared to the elements ofmatrix P. The location of the star within the field of view isdetermined by the point within the field of view that gives the minimumEuclidian distance between the measured output and the characterizationinformation. Because the brightness of the imaged star is notnecessarily the same as the light source used for the characterizationof the imager, a multiplicative factor must be added in to thecomparison between matrix P and the measured signal. The correct starbrightness and location within the field of view are determined byminimizing the sum squared error, represented by Equation (1) below.e=Σ _(i)[M _(i) −P _(i) b]²  (1)

where M_(i) is the measured output, P_(i) is the predicted output, and bis a scaling factor for the brightness of the star. In one exampleembodiment, solving for the constant b can be performed as follows:given two vectors, M and P, we desire the constant b which minimizessum(M_(i)−bP_(i)){circumflex over ( )}2. We can get the minimizing b bytaking the derivative with respect to b and setting it equal to zero:sum(−2P_(i)*(M_(i)−bP_(i)))=0. Dividing both sides by −2 anddistributing gives sum (P_(i)*M_(i)−bP_(i){circumflex over ( )}2))=0.Breaking apart the sum and pulling b out:sum(P_(i)*M_(i))−b*sum(P_(i){circumflex over ( )}2))=0. Rearranging tosolve for b: b=sum(P_(i)*M_(i))/sum(P_(i){circumflex over ( )}2)).

These methods can be used predict the performance of a device based onnumerical calculations of device performance, and can be used tocharacterize a fabricated device based on measured output data.

Multiple stars can be located using the compressive/transform imager asa star tracker and using the process described above. While looking forthe location of one star within the field of view, other stars in thescene add Gaussian noise. For example, if there are 7 stars in thescene, a correct guess of star location will have the noise from 6 starsbecause the output of one star matches the corresponding values in thematrix P. An incorrect guess, however, will have a noise level from 8stars due to all 7 stars in the scene, as well as the incorrect guess ofthe star location. This process can be used to find all stars in a sceneor background subtraction can be used to remove noise due to stars thathave already been found. A larger number of output ports, N_(out),provides better differentiation, especially for larger number of starsor sources or lights in the scene. The number of stars or features, k,that can be located within the field of view is proportional tolog(N_(out)). The probability of correctly locating the positions of asmall number of stars or features is higher for small k than large k.

The runtime of the algorithm is [O(n² k log(n))], where k=the number ofstars or features in the scene. Any additional optimizations, forexample, taking into account the brightness of the star through aleast-squares optimization, adds a constant factor C to the runtime ofthe image extraction algorithm, which leads to a generalized runtime ofthe algorithm of Cn² k log (n).

Generally, the performance of typical star trackers is adverselyimpacted by chromatic aberration. The compressive/transform imagerdisclosed herein can take into account the spectrum of a star in thefield of view. This additional correction requires characterizing thesensor for a series of different wavelengths. Given the black-bodyspectrum of stars, the spectrum will be approximately linear within abandwidth of several 100 nm in the infrared spectrum. Equation (1) abovecan be expanded to take into account the slope of the spectrum, as shownin Equation (2) below:e=Σ _(i)[M _(i)−Σ_(j) P _(ij)(aX _(j) +b]²  (2)

where aX_(j)+b represents to brightness of the star as a function ofwavelength, X_(j).

Runtime of the algorithm is increased by a constant factor by addinganalysis of the star's spectrum. The term representing brightness as afunction of wavelength can also be represented by a higher orderpolynomial. This introduces multiple solutions and further increasesruntime of the algorithm, but can be used to provide more detailedinformation about the spectrum of stars within the field of view.Correcting for the spectrum of the star becomes more important the morematrix ‘P’ varies as a function of wavelength and the more the spectrumof the star or light source varies as a function of wavelength. Thetechniques described above can be implemented to use a flatcompressive/transform imager as an integrated, on-chip spectrometer.

Alternatively, the transfer matrix of the imager can be recovered duringcharacterization. The input scene, I_(scene), is mapped to an outputvector (signal at the output ports of the imager), I_(measured), by atransfer matrix, M_(transfer), by the relationshipI_(measured)=M_(transfer)I_(scene). If the transfer matrix is known, thescene can be recovered by I_(scene)=M_(transfer) ⁻¹I_(measured). Thetransfer matrix will be size N_(out)×[N×N]. M_(transfer) can berecovered using two methods. A bright point source can be scanned acrossthe field of view, essentially producing I_(scene) as a single brightpixel and no signal elsewhere, which recovers the columns ofM_(transfer) one at a time. The imager can also be illuminated by alarge number of random (independent) input scenes to produceuncorrelated pairs of matrices I_(scene) and I_(measured). The number ofinput scenes is >N_(out)×N×N. This means a greater number of randominput scenes than point source locations is required, however, theintegration time for each scene in this calibration method is shorterbecause approximately half the scene has signal, rather than a singlepoint. A least-squares best-fit M_(transfer) or M_(transfer) ⁻¹ can becalculated from this information. M_(transfer) must be square to beinvertible, meaning that this method of imager characterization cannotbe done when output ports of MMIs in the compressive imager are dropped.As used in this disclosure, calibration and characterization may be usedinterchangeably.

Accounting for wavelength in the calibration can be done using severalapproaches. One example is to account for wavelength within the transfermatrix described above. In this case, resolution is sacrificed (by aconstant factor of N_(wavelengths)) in exchange for wavelengthinformation because the transfer matrix has a constant sizeN_(out)×[N×N]. Alternatively, the entire calibration process andtransfer matrix recovery must be completed for each wavelengthindependently. This results in N_(wavelengths) transfer matrices.

Any mathematical basis and transform can be used to recover scenes fromthe output of a compressive/transform imager, so long as the scene issparse in the chosen basis. Discrete sine and cosine transforms are agood choice for a general compressive/transform imager because naturalscenes tend to be sparse in a discrete cosine basis and related bases(e.g. Haar or Fourier). The outputs of the imager are used to determinewhich frequency components are present in the scene. The number offrequency components or features that can be recovered from the scenerelates to the number of output ports the imager has (i.e. c k log(n),where k is the number of features and c is a constant factor). Thismethod can improve the recovery of general scenes.

At least some embodiments of the present disclosure can result inimproved utility in general imaging applications (i.e. imaging complexscenes). Complex scenes require the ability to collect a larger numberof unique data points with the imager. This can be achieved byincreasing the number of input ports, which increases the size of theimager and complexity.

An alternative approach is to “re-phase” the input array. Phase shifterson each input port (e.g., heaters to shift the refractive index in thewaveguides and total phase accumulated by a signal from that port beforeit reaches the first mixing region/MMI) can be used to reconfigure therelative phases of signals from different input ports before they getmixed. This will change the transfer matrix of the device and lead to adifferent correlation between phase front angle relative to the imagerand signal intensities measured at the output ports. The amount ofvariability introduced by phase shifters is maximized if each phaseshifter can introduce an arbitrary phase shift of −π to +π (or 0 to+2π). Each phase configuration with the same imager provides additionalinformation. Taking data with a large number of phase configurationsenables recovery of more complex scenes. The number of phaseconfigurations is preferably between 4 and 256. The number chosen willbe determined by a trade-off of image quality (e.g. sharpness, noise,contrast, dynamic range) and exposure time. A larger number of phaseconfigurations improves image quality, but increases time required totake the image.

In order to make use of “re-phasing” of the imager, appropriatecalibration should be completed. There are several options. For example,calibration can be performed as described herein for each phaseconfiguration of the imager, resulting in a separate matrix P for eachPhase configuration. Another option is to determine the transfer matrixof the imager, M_(transfer). Once M_(transfer) is known it can bemultiplied by the matrix representing the phase shifts applied to eachinput port to determine the transfer matrix for any arbitrary phaseconfiguration.

FIG. 13 is a representative schematic of an array 1310 of 8×8 inputports (e.g. grating couplers), each of which has a phase shifter 1340 oneach aperture 1300 (e.g., grating coupler) capable of a phase shift of0-2π (e.g., thermal phase shifter). Waveguides 1320 are routed from eachinput region through either a series of mixing regions (MMIs) or atleast one large MMI/mixing region 1330 (or several layers of MMI's)where signals from all 64 input ports are mixed. Path lengths should bematched with a tolerance of approximately one wavelength. At the end ofthe MMI, which mixes all signals together, all output ports are kept androuted to detectors. Alternatively, the MMI can be terminated at theedge of the chip/mixing region and imaged directly with a detector (e.g.a detector array bonded directly to the end of the MMI).

FIG. 14 shows simulation results of a random imager which makes use ofre-phasing. The “original object” or “input” 1410 is what the devicetakes an image of. The recovered image 1420 is shown as a function ofthe number of apertures in the imager (8×8, 16×16, 32×32) and the numberof re-phasing steps (1, 4, 16, 64, 256). The image quality improves withthe number of input apertures as well as the number of re-phasing steps.Above approximately 256 re-phasing steps, the image quality plateaus. Inthis case, the simulation assumes all output ports of the device arekept. The image quality recovered by an imager making use of re-phasingis limited by the resolution of the imager. This is a function of thesize of the input array (number of elements and element spacing) andrepresents a fundamental limit for all imaging systems, regardless ofconfiguration (e.g. a CMOS imager with 8×8 array of pixels at the focalplane of a lens would recover a similarly blurry image as the 8×8 randomimager with re-phasing).

Re-phasing allows simplification of the imager without loss ofinformation. Dropping output ports from MMIs can simplify the layout andfabrication of a random imager, but leads to loss of information.However, re-phasing can be used to regain this information. For example,a random imager with 32×32 array of input ports with 512 output ports(half as many output ports as input ports) can recover as complex ascene as a 32×32 array of input ports with 1024 output ports, so long astwice as many re-phasing steps are used and the combination of phases issufficiently unique.

In another example embodiment, an amplitude adjustment can be made priorto directing light to the mixing region rather than a phase shift. Anamplitude adjustment can similarly modify the transfer matrix of theimager, and this modification can be represented by a complex matrix.

The compressive/transform imager described herein can greatly reduce thenumber of waveguides needed. This makes it possible to implement thecompressive/transform imager in a single layer of waveguides, or in asmall number of layers of waveguides. Multi-layer designs can also beimplemented without the need for vertical optical interconnects, greatlysimplifying design and fabrication. Use of the compressive/transformimager requires characterization, which introduces the benefit thatallows effects resulting from fabrication variations to be taken intoaccount, making the imager robust to variability between the design andthe actual fabricated device.

FIGS. 15A-15C show examples of a Rotman lens that can be used as themixer in a compressive/transform imager, according to embodiments of thepresent disclosure. In a non-limiting example, a Rotman lens 1505 can beused as a true time delay device. In the example shown in FIG. 15A, theRotman lens 1505 is connected to a number of light collection apertures1501 (e.g. gratings), and path matching 1503 is ensured between allapertures and the input port of the Rotman lens. A number of outputports 1507 can be connected from the Rotman lens to one or moredetectors. In the Example shown in FIG. 15B, the input waveguides 1511are spaced a distance 1513 of between <λ and 2λ between one another. Theinput ports 1511 can be tapered waveguides configured to reducereflections as light enters the multimode mixing region of the Rotmanlens 1505. Output waveguides 1517 can also be spaced a similar or samedistance apart as the input waveguides. Similar to MMIs, waveguides,tapers, and mixing regions can be made of a core guiding material with arefractive index n_(core)>n_(cladding). In the example shown in FIG.15C, an incoming phase front 1520 arrives at the light collectionapertures of the Rotman lens and the light is focused on a particularoutput 1521, which ensures path matching for signals collected by allapertures.

FIGS. 16A-16C show schematics of a single Rotman lens used for a 2Daperture array, according to embodiments of the present disclosure. Inthis example embodiment, a 3×3 2D array of apertures 1601 are directedto a single Rotman lens 1605, as shown in FIG. 16A. Each output 1607 tothe detectors corresponds to the position of a source in the field ofview 1611 shown in FIGS. 16B-16C. Thus, a first light source at location1613 within the field of view 1611 will result in a signal output 1615to a particular detector. This is a different response from the signaloutput 1625 caused when a second light source at location 1623 isdetected. In this way, a single Rotman lens can be used to determinesource location in two dimensions.

FIG. 17 shows a schematic of a multi-Rotman lens system, according to anembodiment of the present disclosure. In this example embodiment, afirst Rotman lens 1701 is connected to apertures arranged in onedimension for determining position of a source along the y-axis in ascene, while a second Rotman lens 1703 is connected to aperturesarranged in one dimension for determining position of a source along thex-axis in a scene.

FIG. 18 shows a schematic of another multi-Rotman lens system, accordingto an embodiment of the present disclosure. In this example embodiment,more than two arrays of apertures can be tiled in order to improve fillfactor for light collection and provide more information to recover morecomplex scenes. This example shows a set of eight Rotman lenses 1801connected to eight arrays of apertures. One skilled in the art willappreciate that more or less arrays of apertures, or configurations ofarrays, can be implemented.

FIG. 19 shows a schematic of another multi-Rotman lens system used fordetermining light source location, according to an embodiment of thepresent disclosure. In this example embodiment, two 1D arrays ofapertures feed into a first Rotman lens 1905 and a second Rotman lens1907. A light source detected at location 1903 within the field of view1901 will be detected at the first Rotman lens 1905 indicating itslocation in the y-axis, and at the second Rotman lens 1907 indicatingits location in the x-axis. This will lead to an output response 1911indicating the light source's location in the y-axis, and an outputresponse 1909 from the second Rotman lens 1907 indicating the lightsource's location in the x-axis.

Similar to the example discussed above in reference to FIG. 2, theRotman lens can also be implemented in a staged system where severalRotman lenses are used as mixers at numerous stages. In one exampleembodiment, given a 2D array of grating collectors, each column can feedinto a separate Rotman lens, the outputs of which all feed into a singlelarge Rotman lens. The outputs of this structure then feed intodetectors. One skilled in the art will appreciate that such a system canhave as few or as many stages of Rotman lenses as required by a givenembodiment. As will be appreciated, the FIGS. are not necessarily drawnto scale, and the path matching of waveguides is not shown in all theexamples shown in FIGS. The Rotman lens can also be implemented in alayered embodiment, as discussed in reference to FIG. 7 above, whereinthe waveguides and mixing regions (i.e. the Rotman lenses) can bestacked in a number of layers.

Rotman lenses fall under a class of devices comprised of an array ofinput waveguide ports, a mixing region, and an array of output waveguideports, where the input ports, output ports, or both are arranged in acurved geometry. Similar devices include the “Rotman Lens”, “BootlaceLens”, and “Parabolic Bootlace Lens”. For simplicity, throughout thisdisclosure the term “Rotman lens” is used to describe all devices underthis class, including Bootlace lenses, Parabolic Bootlace lenses, etc.The concepts for their use in integrated photonic imaging devices applyto all types of these devices. These devices are characterized in moredetail in the publication: Kushwah, R. P. S., & Singhal, P. K. (2011).Comparative Analysis of Parabolic Bootlace Lens and Conventional LensApproach. Journal of Electromagnetic Analysis and Applications, 3(2),65-68. https://doi.org/10.4236/jemaa.2011.32011

FIGS. 20A-20B show two schematics of input and output spacing forexample Rotman lenses, according to an embodiment of the presentdisclosure. In one example embodiment, the device has input/outputseparations of 1 wavelength. A non-limiting example of such a devicewith 15 inputs/outputs is pictured in FIG. 20A. The length and width ofthe device will be equal and have value “num_outputs*separation”. Adevice with fewer inputs (antennas) than outputs (detectors) iseffectively the same as a device with the same number of inputs andoutputs, but with the unused input ports deleted. For example, a 5input, 7 output device is just a 7 input, 7 output device with, forexample, the left and right-most input ports deleted. The device widthis trivially derived from the separation times the number of ports. Thelength is equal to the width because going any smaller produces phaseerrors that get larger quickly, and going larger is wasteful. Thecircles defining the input and output curves have radius equal to thelength, as otherwise they wouldn't focus onto the center of the oppositeside. The corresponding parabola (which approximates the circle) isdefined by y=x{circumflex over ( )}2/(2*height). Note that the curves inthis example are equal in curvature.

Increasing the separations results in a device that is wider but alsolonger. Doubling only one of the separations doubles the length, asshown in the example of FIG. 20B. This means doubling both (i.e.“scaling up” the device) would quadruple the length(doubled_input_and_output_spacing.png). This generalizes, so triplingthe width multiplies the length by 9.

FIGS. 21A-21B show intensity patters for two Rotman lens devices,according to embodiments of the present disclosure. Increasinginput/output separations (and consequently device width and length)causes the device to become less round and more square, as theparabolas/circles on either side flatten out to increase their focaldistance to reach the opposite side. The flattening produces twointeresting effects: one is reduced phase error, the other is that oneside may be flattened without introducing significant phase error asit's basically already flat. The second effect is visible in theintensity pattern of the devices in FIGS. 21A-21B. The smaller deviceshown in FIG. 21A has spacing/separation of 1 wavelengths, while thelarger one shown in FIG. 21B uses 2 wavelengths. The ratio of the depthof focus of the beam (brighter region where signal is focused, near thehorizontal distance value 0) to the height of the parabola is clearlyworse in the device of FIG. 21A. In other words, the focused portion ofthe beam (as it scans across the FOV) would intersect every part of thelarger device's flattened output shown in FIG. 21B, but not the smallerdevice's flattened output shown in FIG. 21A. The result is thatflattening the smaller device's outputs introduces blur to some off-axisbeam positions, as they're significantly out of focus. This is analogousto the short depth of focus of a short focal length (low F/# or highnumerical aperture) lens versus a longer depth of focus of a long focallength (high F/# or low numerical aperture) lens.

What is claimed is:
 1. A transform imager comprising: a chip; aplurality of input ports formed in the chip for coupling light to awaveguide array; a waveguide mixing region configured to receive lightfrom the waveguide array; and a plurality of detectors for receivingoutputs from the waveguide mixing region, the waveguide mixing regionhaving some outputs that are terminated, such that the plurality ofinput ports has a number greater than the plurality of detectors,wherein the waveguide array is a multi-mode H-tree array configured tomaintain substantially equal path lengths between the plurality of inputports and the plurality of detectors.
 2. The transform imager of claim1, further comprising: a micro-lens array formed on the chip forcollecting light into the plurality of input ports.
 3. The transformimager of claim 1, further comprising: a randomly spaced, sparselyspaced, or non-uniformly spaced input array for collecting light intothe plurality of input ports.
 4. The transform imager of claim 1,wherein the plurality of input ports includes a grid of N×M regularlyspaced input ports.
 5. The transform imager of claim 1, wherein thewaveguide array and waveguide mixing region are made of a materialselected from the group consisting of a silicon, silicon nitride,sapphire, titanium dioxide, diamond, silicon carbide, chalcogenideglass, and plasmonic structures.
 6. The transform imager of claim 1,wherein each of the plurality of input ports is an aperture or a gratingcoupler.
 7. The transform imager of claim 1, wherein the waveguide arrayand a plurality of initial waveguide mixing regions are formed in aplurality of layers.
 8. The transform imager of claim 7, wherein theterminated outputs of the initial waveguide mixing region are randomlyor irregularly spaced.
 9. The transform imager of claim 7, furthercomprising a final waveguide mixing region configured to receivenon-terminated outputs from the plurality of initial waveguide mixingregions and scramble information from the non-terminated outputs beforecoupling them to the plurality of detectors.
 10. The transform imager ofclaim 9, wherein the final waveguide mixing region is a compressiveimager wherein at least some of the outputs of the final waveguidemixing region are terminated prior to detection by second plurality ofdetectors.
 11. The transform imager of claim 1, wherein the waveguidemixing region is a multi-mode Rotman lens configured to focus lightreceived from the first plurality of input ports.
 12. A method of makinga compressive transform imager of claim 1 having n number of input portssuitable for detecting a number of expected features k in a scene,comprising: forming a chip having n input ports formed on the chip forcoupling light to a waveguide array; forming a waveguide mixing regionconfigured to receive light from the waveguide array; coupling aplurality of detectors to the waveguide mixing region for receivingoutputs from the waveguide mixing region; and terminating some of theoutputs from the waveguide mixing region, wherein a number of terminatedoutputs is proportional to k*log(n/k).
 13. The transform imager of claim1, wherein the first plurality of input ports includes n number of inputports for detecting a number of expected features k in a scene, and anumber of terminated outputs is proportional to k*log(n/k).
 14. Atransform imager comprising: a chip; a plurality of input ports formedin the chip for coupling light to a waveguide array; a waveguide mixingregion configured to receive light from the waveguide array; and aplurality of detectors for receiving outputs from the waveguide mixingregion, the waveguide mixing region having some outputs that areterminated, such that the plurality of input ports has a number greaterthan the plurality of detectors; wherein the first plurality of inputports includes n number of input ports for detecting a number ofexpected features k in a scene, and a number of terminated outputs isproportional to k*log(n/k).
 15. The transform imager of claim 14,further comprising: a micro-lens array formed on the chip for collectinglight into the plurality of input ports.
 16. The transform imager ofclaim 14, further comprising: a randomly spaced, sparsely spaced, ornon-uniformly spaced input array for collecting light into the pluralityof input ports.
 17. A transform imager comprising: a chip; a pluralityof input ports formed in the chip for coupling light to a waveguidearray; a waveguide mixing region configured to receive light from thewaveguide array; and a plurality of detectors for receiving outputs fromthe waveguide mixing region, the waveguide mixing region having someoutputs that are terminated, such that the plurality of input ports hasa number greater than the plurality of detectors; wherein the waveguidemixing region is a multi-mode Rotman lens configured to focus lightreceived from the first plurality of input ports.
 18. The transformimager of claim 17, further comprising: a micro-lens array formed on thechip for collecting light into the plurality of input ports.
 19. Thetransform imager of claim 17, further comprising: a randomly spaced,sparsely spaced, or non-uniformly spaced input array for collectinglight into the plurality of input ports.